In a medical field, there have been available a photodynamic diagnosis (PDD) and a photodynamic therapy (PDT). The PDD is a diagnosis by making use of the property of a light-sensitive substance that is distinctively accumulated in tumor bearing tissue and emits fluorescent light when being irradiated with excitation light. In the diagnosis, the light-sensitive substance is given in advance to a living body, and then the tumor bearing tissue is observed by the fluorescent light emitted therefrom. On the other hand, the PDT is a therapy for destroying the tumor bearing tissue using singlet oxygen that is produced by the light-sensitive substance being excited.
In the PDD and PDT, the fluorescent light emitted from the light-sensitive substance in the tumor bearing tissue is required to be imaged and observed with high accuracy under intense illumination light, for example, from shadowless lamps in the operating room. The fluorescent light emitted from the light-sensitive substance is very weak when compared with the illumination light, thus raising the problem that the fluorescence image is buried in the subject image formed by the illumination light.
On the other hand, in recent years, attention has been focused on the method in which the indocyanine green (ICG) is given to the living body as a light contrast agent and excited by being irradiated with excitation light, so that a near-infrared fluorescence image emitted by the ICG is taken and observed in conjunction with the subject image to make a diagnosis. Hemoglobin absorbs light of wavelength shorter than 600 nm and water absorbs light of wavelength longer than 900 nm. On the other hand, the excitation wavelength of the ICG and the wavelength of the fluorescent light emitted by the ICG lie in a wavelength band from 600 to 900 nm in which no absorption by hemoglobin and water takes place. Thus, use of the ICG makes it possible to observe the inside of the living body. However, this method also has the problem that the fluorescence image is buried in the subject image.
To solve such problems, it was suggested that when the subject image including the fluorescent light in the infrared region was separated into RGB components and then the RGB components were re-combined to form a color image, the subject image was separated so as to be richer in the R component, thereby emphasizing the portion being observed that was formed by the weak fluorescent light (Patent Literature 1). However, in this method, since not only the fluorescent light in the infrared region but also the R component light forming the subject image are emphasized at the same time, it is difficult to accurately observe the portion emitting the fluorescent light in the subject image. There is also a problem that a filter for separating the subject image into the RGB components and a mechanism for driving the same are required, leading to an increase in the complexity of the whole system configuration.
It was also suggested that to allow excitation light to be cut and light in the infrared band and the visible light band to be transmitted and received in an imaging device for simultaneously taking a fluorescence image in the infrared band and a subject image in the visible light band, an optical filter having different transmittances for the infrared band and the visible light band should be used to balance between the infrared light image and the visible light image (Patent Literature 2). However, it is difficult to obtain an optical filter which has well-balanced transmittances for the infrared band and the visible light band when the light in the infrared band for forming the fluorescence image is very weak relative to the light in the visible light band for forming the subject image.
On the other hand, it was also suggested that in a light source device used when a subject was illuminated with illumination light including beams of light of the wavelength of excitation light and the fluorescent light wavelength longer than that so as to simultaneously observe both an observation image formed by weak fluorescent light emitted from the portion being observed in the subject and a subject image formed by reflected light from the subject, a white light source should be used to adjust the intensity of the illumination light component in the wavelength region longer than or equal to the fluorescent light wavelength, thereby balancing the quantity of light between the fluorescence image and the background portion (Patent Literature 3). However, it is impossible for this light source device to balance the quantity of light between the fluorescence image and the subject image in the visible light band.
Furthermore, it was also suggested that in an endoscope for observing a fluorescence image in the infrared band and a subject image in the visible light band, a light source device using a white light source was provided with a band limiting rotary filter having a visible light transmitting filter and an infrared light transmitting filter which were disposed so as to divide a circular shape into two parts so that the filter regions to be used were changed for observing fluorescent light and for observing ordinary light, and to simultaneously observe the fluorescent light and the ordinary light, the band limiting rotary filter should be rotated, and an RGB rotating filter on the receiving side should also be rotated in synchronization therewith (Patent Literature 4). However, the band limiting rotary filter cannot change the ratio of the quantity of light between the excitation light for forming the fluorescent subject image and the light in the visible light band for forming the subject image. Furthermore, to simultaneously observe the fluorescence image and the subject image, the band limiting rotary filter of the light source device and the RGB filter of the light-receiving device have to be synchronized with each other, thus leading to the problem of the increased complexity of the whole system configuration.
Note that it is known to provide a light source for excitation light such as an infrared laser separately from a light source for illumination light in order to balance between the excitation light for forming a fluorescence image and light in the visible light band for forming a subject image. However, this case will also lead to an increase in the complexity of the whole system configuration.